The invention relates to a power output stage for switching inductive loads, for example, inductive loads such as fuel-injection valves for internal-combustion engines.
When inductive loads are switched, in particular switched off, a transition to a high voltage state is executed in a very short time. Published European Patent Application 0 680 147 A2 describes a power output stage, in which that high switch-off voltage is limited by Zener diodes in order to protect the switching transistor. When the Zener protection cuts in, a pronounced kink is produced in the time profile of the voltage. That may result in an excessive emission of electromagnetic radiation that can have a detrimental effect on the environment. That is to say other electrical or electronic systems, in automobiles, for example, a car radio may be detrimentally affected by the electromagnetic radiation emission.
In order to reduce the interference from radiation emissions, in the case of switched power output stages with or without Zener protection, it is widely known to retard the time profile of the high voltage transition during the switching-off period. However, that solution retards the entire switching process and results in the switching transistor being exposed to increased power losses and impairment of the switching speed.
It is accordingly an object of the invention to provide a power output stage for switching inductive loads, which overcomes the herein-mentioned disadvantages of the heretofore-known devices and methods of this general type, and which develops a switched power output stage for inductive loads to the extent that interference produced by radiation emissions during switching of loads is substantially reduced without retarding the switching process and without increasing the power loss in the switching transistor.
With the foregoing and other objects in view there is provided, in accordance with the invention, a power output stage for switching inductive loads, comprises a switching transistor having a drain, a gate and a source; a load having a first end and a second end connected in series with the drain of the switching transistor; a voltage source having a first pole and a second pole, the first end of the load connected to the first pole and the source of the switching transistor connected to the second pole; a series connected circuit having a blocking diode and at least one Zener diode connected between the second end of the load and the gate of the switching transistor; and a capacitor connected in parallel with at least one of the at least one Zener diodes.
In accordance with alternate embodiments of the invention, instead of having a capacitor connected in parallel with at least one of the least one of the Zener diodes there can be either a. a resistor, b. a series connected circuit of a capacitor and resistor, or c. a parallel connected circuit of a capacitor and resistor, in parallel with at least one of the least one of the Zener diodes.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a power output stage for switching inductive loads, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.